An image sensor is a device that converts one or two dimensional optical information into an electric signal. Image sensors may be classified into two types: image pickup tubes and a solid image pickup devices. The image pickup tube is widely used in fields related to measurement, control and recognition with adaptation of an image processing technology focused on television. Various applied technologies based on the image pickup tube have been developed.
Image sensors may be further classified into two different types: a metal oxide semiconductor (MOS) type and a charge coupled device (CCD) type. A CMOS image sensor converts an optical image into an electric signal using a device made with CMOS fabrication technology. The CMOS image sensor uses a switching mode that sequentially detects outputs of a MOS transistors for each pixel. Compared to a CCD image sensor, the CMOS image sensor has a more convenient driving mode and is capable of realizing various scanning types. Also, the integration of a signal processing circuit into a single chip makes it possible to miniaturize the CMOS image sensor. Furthermore, the CMOS image sensor helps lower power consumption and reduces manufacturing costs by using a widely compatible CMOS technology.
To increase the sensitivity of such a CMOS image sensor (which may be referred to as a CIS) a micro lens array (MLA) may be used. In the fabrication the MLA, the aberration and area ratio (typically, referred to as a ‘fill factor’) of the lens are relatively important. In the fabrication of an MLA, a thermal reflow method may be used. To improve sensitivity, a method for increasing a ‘fill factor’ is proposed.
In the fabrication of an MLA in the related technology as described above, for example, as shown in FIG. 1a, a spherical lens array may be fabricated using a circular mask. In this fabrication, the fill factor is 0.79, which is insufficient to improve the sensitivity of a CIS. To overcome this insufficiency, as shown in FIG. 1b, the fill factor can be increased to some degree compared to a spherical lens array by fabricating an MLA using a rectangular mask. The exterior shape of the lens is made rectangular to increase the fill factor. However, this creates a problem with aberration generated in the plane of the lens. That is to say, the lens is out of focus.